Atrial fibrillation is a common cardiac arrhythmia. Although not life-threatening, it is associated with stroke and congestive heart failure. Further, patients with atrial fibrillation can experience palpitations of the heart and even dizziness. In short, atrial fibrillation can substantially reduce quality of life.
While drug therapy for atrial fibrillation is available, many patients either are, or become, refractory to such therapy. Drug therapy can also cause undesirable side effects.
Internal cardioversion of atrial fibrillation is also known. This therapy, however, is not widely or commercially available.
External cardioversion of atrial fibrillation is often the last resort for atrial fibrillation patients. However, this therapy generally requires a hospital stay and can be traumatic.
Atrial fibrillation is a progressive disease. In early stages it can be paroxysmal in nature. Many patients with sick sinus syndrome also experience or may develop paroxysmal atrial fibrillation. Research has been conducted to determine if there are predictors of paroxysmal atrial fibrillation. For example, Liu et al (PACE, Vol. 21:79–86) reported that prolongation of P-wave duration is an indicator of interatrial conduction disturbance. They also reported that prolongation of P-wave duration is an indication of sick sinus syndrome and that in those patients, the prolongation of P-wave is associated with an increased incidence of paroxysmal atrial fibrillation. Further, Jordaens et al (JCE 1998:530–534) concluded that it is possible to recognize patients with paroxysmal atrial fibrillation using P-wave signal averaging. They also concluded, however, that its role in the clinical management of patients remained unclear.
Further, Montereggi et al (AJC 1996:266–269) evaluated the correlation between the signal-averaged P-wave duration and the occurrence of paroxysmal atrial fibrillation in hyperthyroid patients with and without a history of atrial fibrillation. They concluded that a P-wave duration cut-off value of 130 ms held specificity, sensitivity, and positive predictive accuracy values of 79%, 85%, and 83%, respectively.
Still further, Cecchi et al (Heart 1997: 44–99) assessed the relationships between P-wave duration and the occurrence of atrial fibrillation in hypertrophic cardiomyopathy. In assessing risk for atrial fibrillation, they reported that P-wave duration greater than 140 ms was associated with sensitivity, specificity, and positive predictive accuracy values of 56%, 83%, and 66%.
While P-wave duration has been found to indicate an interatrial conduction disturbance and provide a predictive tool with respect to paroxysmal atrial fibrillation, other work was also conducted. For example, Stafford et al. (BrHeartJ 1995: 413–418) used spectral analysis.
Spectral analysis was performed on the entire P-wave. The P-wave signals were filtered with a high pass of 15 Hz before Fourier transformation to attenuate large low frequency components. P-wave energy was estimated by summating the energies contained in frequency bands extending from 20, 30, 40, 60 and 80 to 150 Hz. Each was expressed as an absolute value and in an energy percentage ratio. They found that paroxysmal atrial fibrillation patients had more energy in the higher frequency bands of the P-wave and greater spatial velocity.
While the foregoing evidences new diagnostic tools for paroxysmal atrial fibrillation, the use of these tools in an implanted device to trigger atrial arrhythmia prevention therapy has not been addressed. More specifically, as these patients are generally already being treated with an implantable cardiac rhythm management device, it would be advantageous if such a device could also detect an interatrial conduction disturbance and provide atrial arrhythmia prevention therapy responsive thereto in addition to the traditional therapies for sick sinus syndrome. More particularly, it would be advantageous to have an implantable medical device capable of employing additional methods of identifying disease progress towards paroxysmal atrial fibrillation prior to its actual start and then applying pacing therapies to prevent the start of an atrial arrhythmia such as atrial fibrillation. The present invention provides such an implantable medical device.